During the course of the DLC you will liberate a fair few slaves, these guys are all over the place. Once you have finished the story you will need to grind this out a little, it wont take too long.
Best method is to liberate a camp (30 slaves) then liberate a slave ship (90 slaves), once the ship is done a new camp should have appeared, liberate that one, and again by this time a new ship will have spawned....rinse and repeat.